In a hearing aid, acoustical signals arriving at a microphone of the hearing aid are amplified and output with a small loudspeaker to restore audibility. The small distance between the microphone and the loudspeaker may cause feedback. Feedback is generated when a part of the amplified acoustic output signal propagates back to the microphone for repeated amplification. When the feedback signal exceeds the level of the original signal at the microphone, the feedback loop becomes unstable, typically leading to audible distortions or howling. One way to stop feedback is to lower the gain.
The risk of feedback, limits the maximum gain that can be used with a hearing aid.
It is well-known to use feedback suppression in a hearing aid. With feedback suppression, the feedback signal arriving at the microphone is suppressed by subtraction of a feedback model signal from the microphone signal. The feedback model signal is provided by a digital feedback suppression circuit configured to model the feedback path of propagation along which an output signal of the hearing aid propagates back to an input of the hearing aid for repeated amplification. The transfer function of the receiver (in the art of hearing aids, a loudspeaker of the hearing aid is usually denoted the receiver), and the transfer function of the microphone are included in the model of the feedback path of propagation.
Typically, the digital feedback suppression circuit includes one or more digital adaptive filters to model the feedback path. An output of the feedback suppression circuit is subtracted from the audio signal of the microphone to remove the feedback signal part of the audio signal.
In a hearing aid with more than one microphone, e.g. having a directional microphone system, the hearing aid may comprise separate digital feedback suppression circuits for individual microphones and groups of microphones.
WO 99/26453 A1 provides a useful review of methods of feedback suppression in hearing aids.
WO 99/26453 A1 discloses feedback suppression with two adaptive filters connected in series, see FIG. 1.
The first filter is adapted during fitting of the hearing aid to the intended user and/or when the hearing aid is turned on in the ear. This filter adapts quickly using a white noise probe signal, and then the filter coefficients are frozen, i.e. during normal operation of the hearing aid; the first filter operates as a fixed filter.
The first filter models those parts of the hearing aid feedback path that are assumed to be essentially constant while the hearing aid is in use, such as the microphone, amplifier driving the receiver, and receiver resonances, and the basic acoustic feedback path.
The second filter adapts while the hearing aid is in use and does not use a separate probe signal. This filter provides a rapid correction to the feedback suppression circuit when the hearing aid goes unstable, and tracks perturbations in the feedback path that occur in daily use, such as caused by chewing, sneezing, or using a telephone handset.
The series connection of a fixed filter and an adaptive filter provides a good trade-off between speed and accuracy. A single long filter tends to be slow and/or inaccurate. Further, the fixed filter is an IIR-filter with relatively low processor requirements.
However, in practice the filter coefficients of the fixed filter are determined for each individual user when the hearing aid is fitted to the user by a dispenser or another trained person. This not only requires an additional fitting step, but also fails to capture the true invariant part of the feedback path because the feedback path measured by the dispenser already includes some of the variant parts. For example, the fitting of the hearing aid in the ear canal is included in the invariant part, but it may be subject to changes, e.g. when the hearing aid is re-inserted in the ear.
WO 99/26453 A1 also mentions the possibility of allowing the first filter to adapt slowly to follow slow changes in the hearing aid, such as component drift. However, no further explanation on how to allow the first filter to slowly adapt, i.e. no method of adaptation for the slow adaptive filter, is disclosed in WO 99/26453 A1.